In general, a power transmission apparatus for a four wheel drive vehicle can transmit the driving force of an engine and transmission to the rear wheels and arbitrarily to the front wheels. Thus, it is possible to arbitrarily select a two wheel driving mode where the power transmission to the front wheels is cut off or a four wheel driving mode where the power transmission to the front wheels is achieved. In such a power transmitting apparatus, it has an input member connected to an engine and transmission, an output member selectively connected to selectively driven wheels (e.g. front wheels), and clutch members connected to the output member. The power can be transmitted by pressure-contacting a plurality of driving clutch plates mounted on the input member and a plurality of driven clutch plates mounted on a clutch member. The power can be cut off by releasing the plates.
Japanese Laid-open Patent Publication No. 150517/1990 discloses providing cam members near an output shaft. The output member adds a pressure-contact assisting function to improve the pressure-contacting force of both driving and driven clutch plates during power transmission. Also, it provides a back-torque limiting function to release the clutch plates of the input and output sides when the rotation speed of the output member has exceeded that of the input member. According to this power transmitting apparatus of the prior art, cam members are arranged between the shaft and the clutch member. Thus, the pressure plate (“clutch pressure plate” in the cited reference) is moved toward a direction where both clutch plates are strongly pressure-contacted during power transmission. On the contrary, the pressure plate is moved in an opposite direction where both clutch plates are released when a back-torque is achieved.
However in the power transmitting apparatus of the prior art, since the cam members function as the pressure-contact assistance, and the back-torque limiter is positioned near the shaft as the output member, a problem exists in that the surface pressure applied to the cam surfaces of the cam members would be overloaded. Accordingly, it is necessary to manufacture the cam member from material having high rigidity, such as a forged article, so that it has sufficient durability against the overload. Accordingly, this increases the cost of the power transmitting apparatus. Furthermore, the addition of the cam members increases the number of parts that form the apparatus and reduces the workability of assembling the complicated apparatus which would further increase the manufacturing cost.
The power transmitting apparatus of the prior art is formed with stopper portions to prevent rotation of the pressure plate relative to the clutch member in a state where the pressure plate is mounted on the clutch member. The stopper portions are formed to project from the pressure plate and fit into recesses formed on the inner peripheral wall of the clutch member to limit the rotation of the pressure plate relative to the clutch member. In addition, the outer peripheral surface of the clutch member is formed with splines to fit with driven clutch plates.